Average Daylight Factor

The Average Daylight Factor (DFave) is useful tool for daylighting design. It predicts the brightness of an interior space under daylighting. The concept is best understood by practical experience. It is worthwhile taking time to calculate the DFave in a number of rooms with which you are familiar, and then perhaps to compare your calculations with actual measurements using illuminance meters. This will give a sense of the numbers involved.

The Daylight Factor (DF) defines a constant relationship between a place on the inside of a space and the outside. It is the percentage of the daylight available at an unobstructed place outside which is received at a point inside. [Technically it is the

Less than 2% a room appears gloomy under daylight alone. Full electric lighting is often needed during daytime and dominates the daytime appearance. Sensitive spaces, such as art galleries, may require low DFave, but generally it is preferred to aim higher.

2% - 5% is usually the optimum range of daylighting for overall energy use. Rooms have a predominantly daylit appearance but supplementary electric lighting is needed away from windows and during dull weather. Most room types benefit from this range of DFave, such as offices, shops, sports halls, warehouses and factories.

Above 5% a room is strongly daylit and daytime electric lighting is rarely needed except perhaps to balance illuminance in dark recesses with the general light level, to avoid glare problems. Such DFave’s tend to be found in atria, conservatories, and other large, glazed spaces. In other buildings, with a DFave of 5% or more, unwanted thermal and acoustic effects may arise due to large window areas.

Approximate diffuse transmittance of clean glazing- T

Clear single glazing 0.8

Clear double Glazing 0.7

Low Emissivity Double Glazing 0.65

This should be multiplied by the appropriate Maintenance factor from the table in the Colour section. Values of Glazing Transmittance (T) for other glazing systems can be found in the CIBSE Daylighting and Window Design Guide Code and from manufacturers.

Average Daylight Interior Illuminance

Factors (%)

CIE Standard Overcast Sky Bright Overcast Day

- 5000 lux -12,000 lux

1.0% - 2.0% 50 - 100 lux 120 - 240 lux,

2.5% - 3.5% 125 - 175 lux 300 - 420 lux,

4.0% - 5.0% 200 - 250 lux 480 - 600 lux

Understanding Daylighting of Sports Halls

Designing for Daylight

percentage of the unobstructed outdoor horizontal diffuse illuminance which is received indoors on the working plane and walls below.] The Daylight Factor varies throughout a space, tending to be very high near the windows and rapidly decreasing further from them, hence the use of the Average Daylight Factor (DFave) for approximation.

The DFave, across the horizontal plane in the case of sports hall this would be the floor, can be estimated, for an existing design, by using the calculation shown opposite. Daylighting should be designed to compliment or displace artificial lighting for as long as possible during daylight hours. The Average Daylight Factors (DFave) below give interior illuminances as shown. Aiming for a DFave of 2.5% will ensure that on most occasions the illuminance inside due to daylighting will suffice for recreational activities, and during bright conditions may even satisfy club activities. However, it will be found that most sports bodies will prefer the artificial lighting to be on during critical activities to achieve target light levels.